The present invention relates generally to the filter molding of electrical heating units and more specifically to a filter molding process wherein the heating element is completely embedded in a body of insulating material.
Processes for filter molding an electrical heating unit comprising a heating element and an insulating refractory support are well known in the art. Briefly, such processes as described, for example, in Hesse et al, U.S. Pat. No. 3,500,444 utilize a liquid suspension of any of the well known ceramic refractory fibers. The electrical heating element first is placed against the screen of the filter mold and then a differential pressure created which forces the liquid suspension through the filter screen. The refractory fibers filtered from the suspension are built up on the screen and about the heating element to produce a unit wherein the heating element is completely embedded in the insulating material except for the portion of the heating element direct adjacent the screen.
This portion of the heating element exposed at the surface of the insulating body creates several problems during both the manufacture and use of the unit. For example, during manufacture, it is difficult to position the heating element so that a uniform amount of the heating element is exposed. In use there are many applications where it is not only undesirable, but dangerous to have any portion of the heating element exposed and when manufactured according to the prior art filter molding methods, it is quite possible for the heating element to pull loose during use of the heater. For example, one common alloy for the heating element is a chrome-aluminum-iron alloy. During use, the normal expansion and contraction of the heating element and the grain growth which occurs in such alloys will cause the heating element to pull loose from the supporting ceramic fiber insulation.
The above mentioned Hesse patent teaches that, after filter molding, the exposed surface of the electrical heating element may be protected by overlaying and securing a glass or ceramic cover to the surface of the heating unit. This patent also teaches that an electrical insulation may be supercomposed over the exposed portion of the electrical heating element prior to overlaying the glass or ceramic cover. This method, however, involves extra steps in manufacturing and the difficulty in obtaining the appropriate high temperature bonding agents where mechanical means to attach the cover and electrical insulation are not suitable.
Another proposed solution to this problem is to filter mold a layer of the refractory fiber on the filter mold screen, place the heating element against this layer of fiber and continue the filter molding process to complete the formation of the insulating refractory support. This method too, is not entirely satisfactory in that it is difficult to control the thickness of the layer of refractory fiber initially deposited on the filter mold screen. Further, this initial layer is delicate and easily disturbed when the heating element is placed against it and it is quite possible that the heating element will pierce this initial layer. All of these factors make it difficult to locate the heating element at a uniform depth below the surface of the insulating refractory support, consequently, too much insulating material between the heating element and the surface of the refractory support causes inefficient operation of the heating unit and produces hot spots and subsequent burn out of the heating element. Due to the vacuum forming process, lumping of the fiber in the solution is sometimes difficult to overcome. Therefore, when trying to lay down the initial layer of fiber, prior to inserting the heating element, this lumping will cause an uneven fiber depth. Subsequently, this can cause substantial temperature differentials along the length of the element (hot spots) and poor uniformity over the length of the heating element.
The present invention provides a filter molding method wherein the heating element is completely embedded and fixed in the insulating refractory support at a controlled predetermined depth below the surface of the support.
One example of the type of heater which maybe made by the method of the present invention is a sleeve heater having integral vestibules. Sleeve heaters are typically split cylindrical heaters which are placed about a pipe or line to be heated. Each half or third, etc. of the cylinder heretofore consisted of three parts, a central portion which contained the electrical heating element and sized to accommodate pipes of various diameters and the end portions or vestibules sized to closely fit one pipe diameter. The three portions forming one half of the sleeve heater together with three similar portions forming the other half were assembled in place about the pipe to be heated.
In the sleeve heater of the present invention the entire half or third, etc. of the sleeve heater is filter molded as a unit comprising the central portion with integral end portions or vestibules, the vestibules being easily adapted to accommodate pipes of various diameters within a range acceptable by the central portion.